After a period of indifference, in June 1939 a demonstration of the Power Jets WU was made before a delegation of the Air Ministry, notably Dr Pye, Director of Scientific Research. The demonstration was so successful that the Ministry quickly arranged to buy the engine to give Power Jets working capital, lending it back to them for testing.

At the same time, a contract was placed for a "flight engine", the W.1.[4] Unlike the Whittle WU, that began bench testing in 1937, the W.1 was a symmetrical engine designed to facilitate, after development, installation in an aircraft. The W.1 used a double-sided centrifugalcompressor of Hiduminium RR.59 alloy,[5] reverse-flow combustion chambers and a water-cooled axial-flow turbine section; this was later modified to use air-cooling. The turbine blades were of Firth-VickersRex 78, a stainless steel developed under Dr. W. H Hatfield.

As development of the new design dragged on, it was decided to build a test unit "early engine" using any components that were deemed unairworthy along with test items. This was assembled to become the one-off W.1X.This officially unairworthy unit powered the Gloster E.28/39 on a short 'hop' during taxiing trials in April 1941, with flight trials taking place a month later with a definitive W.1 engine.[6]

After a visit to England in 1941, General Henry H. Arnold arranged for the W.1X to be shipped to the U.S, along with drawings for the more powerful W.2B engine. The former became the prototype of the General Electric I-16 and by April 1943 had been developed to produce 1,650 pounds force (750 kgf).[7]